It is known in the audio signal processing art to adjust sound volume by applying the audio signal to a tapped resistor divider and to select, as output signal, signal from one of the taps using a transistor switch arrangement. Typically the transistor switch arrangement is realized with an array of metal-oxide-semiconductor field effect transistors (MOS-FET's) or complementary MOS-FET's i.e. CMOS-FET's. With this arrangement it is possible to achieve a coarse and fine adjustment e.g. a level adjustment from zero to-110 db, in steps of arbitrary width, e.g. in steps of two db. The CMOS-switches however have a detrimental property, which results in the generation of clearly audible clicks at each change of level. The clicks are developed for several reasons. The CMOS-switch arrangement is constructed in integrated circuit form, consisting of e.g. field effect transistors (FET) controlled by binary codewords for enabling selected ones of the transistors. The switching transistors possess unavoidable stray capacitances between the respective control or gate electrodes and the source-drain-electrodes. Also the switching transistors contain capacitances coupled between the logic circuitry which decodes the binary control-code into transistor gate control signals. There also exists unwanted capacitances between the supply voltage bus, ground and the signal path. Because of the existence of these capacitances, during incremental adjustments of the signal amplitude by selectively sequentially enabling a succession of MOS-FET switches, a series of narrow voltage spikes are imposed on the audio signal. These spikes result from the transitions of the transistor control signals being capacitively coupled to the signal path. The spikes have a magnitude approaching the magnitude of the supply potential. Even if the voltage spikes have a duration in the range of microseconds, they will be integrated by the low pass effect of the audio output amplifier, as well as the physiology of the human ear, to produce an audible clicking in the reproduced sound.
It is an object of the invention to provide a switched audio amplitude control system free of disturbing clocking noises.